Tt-Olefin complexes

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

The TT-olefin complex is analogous to the complex which Green and Nagy 40) reported for the protonation of the a-2-butenyIiron complex ... 

Recently, Rooney (80) expressed the view that the a,j8 exchange process involved tt olefin complexes and asserts that this explains the pattern of exchange on a palladium film of deuterium with 1,1-dimethylcyclo-butane. Its failure to exhibit appreciable multiple isotopic exchange was attributed to the difficulty of forming a tt olefin complex because of the strain in cyclobutene. 

First, TT-olefin complexes are formed by virtually all of the Group VIII metals, and the strength of the olefin-metal bond may vary considerably depending upon the other ligands present in the complex. All of these metals adsorb olefins and are active in olefin hydrogenation. 

The Tt-allyl ligand bonded to transition-metal cations can be susceptible to nucleophilic addition reactions either on a terminal or on the central carbon atom. Regioselec-tive addition to a terminal carbon of an allyl ligand produces a metal-tt-olefin complex (see 5.8.2.3.10), whereas regioselective addition to the central carbon leads to metallacyc-lobutane formation ... 

TT-Olefin Complexes from Nucleophilic Attack on Ti-Aliyi-Metat Complexes. 

The palladium chloride-coppeifll) chloride couple (28, 29) used industrially in the Wacker process oxidizes olefins to carbonyl compounds. Experimental kinetic and isotope effect data (30) seem to indicate that a TT-olefin complex is initially formed in a series of preequilibrium steps. The rate-determining step is postulated to be a rearrangement of the TT-olefin complex to a cr-complex followed by the final breakdown of the cr-complex to products. Figure 13 depicts the widely accepted Henry mechanism (31). 

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

Investigation of Tt-olefin metal complexes with (Z)-2-butene or similar C2v olefins has shown that the energy barrier to the rotation is rather low67,69,70) and that therefore the two conformers must rapidly reach the equilibrium even at room temperature. 

Thus, we have shown that, in addition to the known methyl and chloride ancillary ligands in zirconocenes, amido ligands can also be activated by strong Lewis acids such as MAO, producing cationic complexes active in the polymerization of tt-olefins. 

Metal 77-cyclopentadienyls somewhat resemble the rr-allyl complexes. Initially, when the nature of the metal-allyl bond was not sufficiently clear, the similarity was emphasized many times [see the review by E. O. Fischer (425)]. The similarity shows itself, for example, in the equal antisymmetric C—C stretching frequencies (1640 cm ), which indicate that the force constants, hence the bond orders, are close. The central rr-allyl proton absorbs in the same NMR region as do the protons of coordinated cyclopentadienyl. Both ligands display the symmetrical sandwichlike bond with their metals. Today, however, it is clear that the complexes differ significantly in type, the difference being associated first of all with the fact that TT-allyl complexes are much more efficient than 77-cyclopentadienyls at transforming to o-allyl or 77-olefin compounds. This may be due to the difference between the delocalization energies, 2.472 and 0.828 eV for cyclopentadienyl and allyl anions, respectively (426). 

Chatt and Duncanson extended this concept to platinum-olefin complexes such as Zeise s salt 29), In platinum-olefin complexes, the o--type bond results from a filled tt orbital of the olefin overlapped with a vacant 5d6s6p orbital of platinum, while the 7r-type bond results from a filled 5d6p orbital of platinum overlapped with the antibonding olefin tt orbital (V). 

Olefin complexes can often be prepared in solution by adding the olefin to a soluble Pd(II) salt. Thus NaaPdCl, in HOAc, will absorb ethylene reversibly to give solutions of a different color than the original solution of the Pd(II) salt (106). In some cases the intermediate tt complex can be detected in catalytic systems. Thus, in the oxidation of ethylene to acetaldehyde, formation of the intermediate tt complex, according to the following equilibrium... 

From previously reported studies then, several different products are possible. The initial attack by the oxygen moiety may apparently be vinylic (on either of the two carbons of the double bond) or allylic (on the carbon next to the doubly bonded carbons). Distinction must be made between allylic attack as described here and allylic products which can arise either by true allylic attack or by vinylic attack followed by olefinic isomerization. Thus it is not clear whether such products as 2-hexen-l-yl acetate(II) (58) have been formed by vinylic attack upon hexene followed by olefinic isomerization, by olefin isomerization of hexene to 2-hexene followed by allylic attack, or by some type of synchronous mechanism in which oxygen attack and olefin isomerization occur simultaneously. This last possibility could be visualized as involving some type of 7r-allylic complex (Reaction 2). This involvement of TT-allylic complex can be ruled out only in the production of isopropenyl acetate from propylene since a mechanism such as this followed by olefin isomerization could not be used in that case. For the butenes and higher... 

The ready hydrogenation and isomerization of methyl oleate and palmitoleate with Fe(CO)s confirm the results of Ogata and Misono (18) with monounsaturated aliphatic compounds. In the isomerization of monoolefins Manuel (15) suggested the occurrence of equilibria involving either 7r-olefin HFe(CO)3 and a-alkyl Fe(CO)3 complexes, or TT-olefin Fe(CO)3 and 7r-allyl HFe(CO)3 complexes. The formation of olefin-iron tetracarbonyl complexes has been reported (19). The reaction of butadiene and Fe2(CO)9 has been observed to lead to the formation of butadiene-Fe(CO)4 and butadiene-[Fe(CO)4]2 complexes in which one or both double bonds are pi-bonded to the iron (16). A mechanism involving both monoene-Fe(CO)4 (I) and allyl-HFe(CO)3 complexes (II) is postulated for the isomerization of methyl oleate (Scheme II) and for its homogeneous hydrogenation. 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(ii). in such palladium(ll) olefin complexes (1), which are square planar d complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled tt-orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin tt -orbital (back donation), leads to an electrophilic activation of the alkene. ... 

William Christopher Zeise (1789-1847) was a Danish apothecary and professor in Copenhagen, Denmark. He synthesized the first metal-olefin complex by serendipity (this term is explained in Chapter 4), when he treated platinum(IV) chloride with ethanol and potassium chloride K[PtCl3( -C2H4)], sal kalico-platinicus inflammabilis , cf [73], TT-Complexation of olefins at transition metals nowadays comprises a key feature of homogeneous catalysis in terms of olefin activation, with the Wacker-Hoechst process being a prominent example (cf. Section 2.4.1). 

In the first stage Lewis acids are absent and further hydroeyanation of the monoolefm products 3-PN 40 and 2M3BN 41 does not readily oeeur. The monoeyanation of butadiene is similar to HCN addition to olefins. An individual feature of hydrocyanation of conjugated dienes is the intermediate appearance of TT-allylic complexes 43, which participate in the successive carbon-carbon coupling. Equations (12) and (13) demonstrate the reaction of butadiene with the hydrido-nickel complex 42 leading to formation of nitrile 40 (a) and explain the generation of byproducts, i.e., the branched nitrile 41 via an alternative pathway (b) [68-70]. 

The nature of the bonding in Zeise s anion and other 7 -olefin complexes is illustrated in Fig. 18.2. Without the push-pull mechanism, the tt electrons of the olefin would have little or no tendency to allow themselves... 

As mentioned in the introduction, TT-bonded Ag-olefin complexes are paradigms in the thermochemistry of organometallic species. Nonetheless, there are comparatively few relevant quantitative data. 

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